Sound absorbing structure



April 13, 1937. 11 M PRUDDEN 2,077,155 i SOUND ABSORBING STRUCTURE Filed NOV. l5, 1935 iff.

Patented Apr. 13,l 1937 UNITED STATES PATENT OFFICE SOUND ABSORBING STRUCTURE Theodore M. Prudden, Hingham Center, Mass.

Application November l5, i935, Serial No. 49,904

(Cl. iSd-44) 8 Claims.

- This invention relates to an acoustic material or panel for the purposes of insulating a room or other enclosure and especially the interior of an automobile body against entrance of sound thereinto and for absorbing sound within the enclosure so as to eliminate undesirable echoes and reverberations or others results in the' enclosure and is intended as an improvement on the material of my copending application Serial No. 675,- '768, led June 14, 1933.

The panel of the aforesaid application comprises avplurality of superposed, spaced, untensioned or loosely-draped sheets interposed between a sound reflectingv backing sheet and a sound permeable cover sheet, the spacing of the loosely-draped fabrics or layers permitting each layer to move in response to sound waves impinging thereupon to absorb the sound and to convert the energy thereof into a not readily audible movement of the sheets.

'I'he present invention is directed to the same general type of acoustic material and has for an object an improved manner of obtaining the desirable spacing between the various sound absorbing sheets.

In the present invention instead of using superposed spacer members interposed between the sheets at marginal portions thereof, as in the aforesaid application, the sound absorbing sheets or layers are formed with'ridges, projections or other suitably elevated portions sulciently distributed and spaced from each other throughout the surface area of the sheets so that the elevations of one sheet engage the adjacent sheet and hold the sheets in suitably spaced relation. The arrangement of the elevations is such that the sheets, and portions thereof, can respond or move individually upon impingement of sound waves thereupon suitably to absorb the sound and convert the sound energy into a non-audible form of energy. Such an arrangement constitutes a further object of the invention.

It is another object of the present invention to obtain the elevations of the absorbing sheets by vcreping or plissing the material of the sheets.

The panel thus formed is mechanically elastic by reason of the formation of the sound absorbing layers and is capable of returning to its initial form following the removal of a compressive force.

A further object is generally to improve the construction and performance of sound absorbing and insulating materials.

Fig 1 is a perspective view of an acoustic panel embodying the present invention.

Fig. 2 is an enlarged perspective view of a section of the panel of Fig. 1 between two lines of retaining stitches.

Fig. 3 is an enlarged sectional detail of the panel taken along lines 3-3 of Fig. l.

Fig. 4 is a plan view of a portion of a sound ab-y sorbing sheet.

Fig. 5 is a sectional detail of the acoustic material of the present invention installed on the oor of an automobile body under the carpet.

As herein illustrated the sound absorbing and insulating panel comprises a relativelyT thick sound pervious front or cover sheet l0, a relatively thin backing sheet l2 and a sound absorbing section it. The main function of the front sheet is for ornamentation and to protect the sound absorbing section and said front sheet comprises a soundv pervious material that reflects, usually, but a small amount of the sound that reaches it and passes to the absorbing section i3 the portion of the sound not reflected and absorbed by it. The front sheet is porous and thereby sound permeable and for the purposes of this invention is flexible so that it can bend without rupture in response to pressures applied thereon that cornpress the absorbing section and is sufficiently elastic to return to its original conguration when the pressure is removed. A sheet of loosely combined material as a felted sheet is satisfactory and the sheet lll herein illustrated is composed of felted brous stock or is made by a paper making process.- A textile felt can be used but costs more. The backing sheet l2 is sound impermeable and is intended to reflect a large part of the sound impinging upon it. A relatively thin kraft paper sheet is quite satisfactory for the purpose. The sound absorbing section i3 comprises a plurality of superposed, untensloned or loosely-draped, spaced, sound absorbing light, thin sheets M of any suitable material capable of vibrating or moving in response to sound waves and of vabsorbing the sound by converting the sound into the inaudible energy of movement of the sheets. lThe sheets are incorporated in the structurepreferably in such manner that they are :tree from energy of the wave not absorbed by one sheet to pass to other sheets. Whereas, in my prior application, the sheets I4 were spaced apart by separate spacer members interposed between the sheets at margins thereof, and sometimes also in the middle of the sheets, so that the sheets were individually vibratorily responsive to the sound waves, in the present invention, the spacing of the sheets is effected in a different manner, so that, for one reason, the collection of sheets, or panel, can better withstand compres` sive forces without mechanical or acoustical damage.

In accordance with the present invention the sheets I4 are spaced apart by suitably providing the surface of each sheet 'I4 with a series of ridges or elevations I6 which are integral with a part of the'sheet and are suitably spaced apart throughout the area of the sheet and engage or bear against confronting portions of adjacent sheets. The sheets can be provided with said ridges or elevations I6 in any suitable manner although 'I prefer to make them by creping or plissing the sheet. In making a crepe textile sheet I4 one set of yarns, as the warp yarns, or a suitable number of the yarns of the set, can be given a high twist with respect to the other yarns so that when the tension thereon is released at the termination of the weaving operation, the high twist yarns will tend to untwist and hence draw up the fabric into a series of ridges or elevations I6'. In plissing, the sheet is printed'in dots or spaced localities with an alkali solution, as caustic soda, which alters the structure of the fibres and causes the puckering of the material and the formation of the ridges or elevations I6. The spacing between these raised portions can be determined by a suitable.

spacing of the high twist yarns, in a crepe textile fabric, and by a suitable spacing of the printing characters, in a pliss sheet. The height of the elevations I6 preferably is materially greater, three or four times, than the thickness of the fabric. The elevations I6 are characterized by being resilient or springy and when flattened by compressive pressure tend to resume their original form when the pressure is removed, so that the sheet remains substantially permanently in the shape imparted to it, so that even when the sheets I4 are laid on the floor of an automobile under a carpet and under foot their resilience will continue for a long time.

'I'he spaced sheets I4 are bound loosely between the front and back sheets by lines of stitching I8, 20 and 22, which pass through the end margins and also the middle of all superposed sheets, the sheets between the stitch lines being free and unattached. The stitchings are loose, being merely for the purpose of retaining the sheets in assembled form, and are not intended to exert any material compressive eect thereon. In the stitching process fingers, not shown, are positioned temporarily under the stitches so as to ensure loose stitches.

A loosely-draped or untensioned thin, light sheet 24 which can be of tissue paper, as a stretched crepe paper sheet, that is,` a crepe sheet that has been stretched to pull out wrinkles, is positioned between the cover sheet I and the first sheet of the collection of sheets I4 and similar sheets 26 and 28 preferably are interposed between the collection of sheets I4 and between the collection and the back sheet I2 respectively. If desired, and for heat insulation purposes, the sheet 28, and also the sheets 24 and 26,*can be aluminum foil. The sheets 24, 26 and 28 preferably are retained in the panel by the middle line of stitching 2li with their opposite ends terminated short of the outer lines of stitching 'I8 and and so are free to vibrate. The functions of the sheets 24, 26 and 28 are the same as in the construction of my earlier filed application. Very little sound is reflected from these sheets. They ordinarily vibrate to the sound waves and transmit the sound waves to other sheets of the panel. The sheets I4 in contact with the sheets 24, 26 and 28 also act as dampeners for the vibrations of the latter sheets. If desired, a loosely woven textile sheet 30 of the nature of burlap is applied over the front sheet II) and is held in place by the lines of stitching. Said sheet 30 is intended to be sound pervious and is of .little importance in reflecting or absorbing sound, its function being mainly to protect the cover sheet As thus arranged the sound irnpinging against the front cover sheet I0 can pass freely therethrough into the interior of the panel. The sound wave setsv in vibration the first crepe sheet 24 which absorbs a part of the sound and transmits the remaining part to the series of sheets I4. 'I'he sound impinging against `these sheets sets them in vibration so that each absorbs a substantial amount of the energy of the sound wave, the remaining energy passed through any sheet being successively absorbed in the successive interior layers of the panel. Any unabsorbed portion of the sound wave impinges against the back sheet I2 and is reflected therefrom and is directed to pass outwardly of the panel through the sheets which absorb substantially the remaining portion of the energy of the Wave. The freely vibratory paper sheets also absorb the energy of and modify the shape of the sound wave and thereby enhance the absorption of sound. 'I'he construction is a good sound insulator since sound that penetrates the structure from the back, or through the back sheet I2 is absorbed as above described. The elevations I6 are sufflciently spaced to keep the sheets I4 apart, except ln the small areas where the elevations contact with the confronting sheets, sufficiently so that each sheet can vibrate or shake, yet the spacing of the elevations is sufficiently close so that after the panel has been compressed together and the compressive pressure removed the sheets will remain separated.

An limportant use of the acoustic material v herein described is in lining the floors and interiors of automobile bodies to prevent the passage of sound thereinto and to absorb sound in the interior.V Fig. 5 illustrates the material ap'- plied upon the floor 32 of an automobile body and under the carpet 34 which preferably is a textile product, that is sound permeable, as a pile carpet, for instance, and not a rubber mat or the like which reflects sound and hinders the passage of theV sound waves to the acoustic material. The acoustic materialis positioned with the back sheet I2 next to the floor 32 and the front sheet I0 immediately under the carpet. As thus arranged sound originating on the outer side of the floor is largely reflected from the back sheet and is prevented from entering the interior of the body. Sound within the body is absorbed by the acoustic material in the manner above described.

The elevations I6 are resilient as has been stated heretofore so that when the carpet 34 is trod upon the elevations will be compressed but will 'tend to spring back to their original position when the pressure is removed and hence restore the proper spacing of the sheets i4, without damage to the material. i

I claim:

1. An acoustic panel including a multiplicity of closely superposed, limp, loosely-draped, light, thin sheets capable of individual vibration, the

` material of said sheets being distorted to provide elevations distributed over the faces thereof, the( elevations of one sheet being loosely engaged with a confronting sheet and separating said sheets.

K2. An acoustic panel including a multiplicity of closely superposed, limp, loosely-draped, light,

thin sheets capable of individual vibration, the material of said sheets being distorted to provide elevations4 distributed in spaced order over the confronting areas of the sheets, the elevations of one sheet being loosely engaged with a confronting sheet and separating said sheets, and at least certain of said sheets being composed of a textile material.

3.An acoustic panel including a collection of closely superposed, limp, loosely-draped, light,

thin, loosely-contacting creped sheets.

4. An acoustic panel including a plurality of closely superposed, limp, loosely-draped, light, thin, loosely-contacting plissd sheets.

5. An acoustic panel including a plurality of closely superposed, limp, loosely-draped, light, thin, loosely-contacting plissd sheets of textile material.

6. An acoustic panelcomprising a, porous sound permeable front sheet, a backing sheet, and a plurality of limp, sound-absorbent, closely superposed, vibratorily-responsive sheets that are loosely-draped, light and thin and are interposed between said -front and backing sheets and are capable of individual vibration, the material of said Vibratorily responsive sheets being deformed to provide resilient spaced elevated portions distributed over the surfaces of the sheets and loosely engaged with confronting sheets and holding said sheets spaced apart.

7. An acoustic panel comprising a porous sound permeable front sheet, a backing sheet, and a plurality of closely superposed, looselydraped, light, thin sheets interposed between said front and the backing sheets and capable of individual vibration, said loosely-draped sheets being formed with integral elevated portions distributed over the surfaces of the sheets and loosely engaged with confronting sheets and holding said sheets spaced apart, and fastening means passing through the similar marginal portions of all of said sheets, said fastening means being loose and exerting substantially no compressive eifect upon the sheets.

8. An acoustic material comprising a flexible sound permeable front sheet, a flexible back sheet, and a plurality of closely superposed, loosely-draped, light, thin, sheets capable of individual vibration and interposed between said front and back sheets and having surface projections that are spaced apart and confront and loosely engage and space apart adjacent sheets, the projections being integral with and composing a part of the sheets and being resilient and having small surface area as compared with the sheet area between the projections.

THEODQ'RE M. PRUDDEN. 

